We found that the father and mother carried the p.Y309C and p.R565W and mutations, respectively, while the two probands inherited both variants from their parents, resulting in compound heterozygous mutations in NAGLU. Therefore, Sanger validation confirmed that these two mutations are inherited from different parents, which fits the known inheritance mode of the Sanfilippo syndrome. The gene discussed is NAGLU; the disease is mucopolysaccharidosis type 3.